An Introduction To The USGS Landsat 7 Image Viewer
By GeoCommunity Staff
The goal of the Global Visualization L7 Image Browser project is to
establish a visualization application
that simplifies the scene selection
process for EDC satellite data by
creating a more efficient method of
searching the massive quantities of
imagery contained with EDC
The USGS recently launched a new tool to quickly access, browse, and view
bands 5,4,3 of Landsat 7 imagery prior to purchase. The Landsat 7 Image Viewer Version 1.1 (http://edcsns17.cr.usgs.gov/NewEarthExplorer/) uses a graphical interface to navigate the global Landsat 7 archive
managed by the U.S. Geological Survey. Users can browse images from the entire earth's surface
for scenes acquired and processed through the previous day. The world has been divided into rectangular regions with each region name being referenced
by the Latitude/Longitude value of their upper left. To zoom in, users are requested to click on a specific region of a 63,424m resolution world mosaic of
the most recent, lowest cloud cover Landsat 7 images.
Image 1: Welcome screen from L7 Viewer Website
Navigating The Globe
Navigating the site is very simple. Visitors are initially presented with a map of the World (see image 1).
Clicking on the image causes an indexed map of a Landsat image mossaic to be displayed for the area highlighted in the
blue rectangle. The index map is made up of imagery from approximately 58 paths and 28 rows. Clicking on the image
results in a more detailed 3x3 image mosiac being called up by a viewing applet (see Image 2 below).
The 3x3 viewer provides the user with horizontal and vertical panning (click on arrows),
a quick jump option enabled by entering path/row values, and a "birdseye" overview showing the current position
with a green dot - the birdseye image is also clickable making for easy movement around the globe (very handy).
Image details provided include the path/row number, % cloud cover in scene, and date of image.
Viewing temporal changes is possible by using the PrevDate/NextDate buttons to go backward/forward in time.
Individuals wishing to view the imagery in even higher resolution can click on the
"show full rez browse" button to get an impressive high res. image displayed in the viewer (see Image 3).
If at any time users require positional information, the latitude/longitude of the cursor is continually displayed in the browser's status bar.
Image 2: 3x3 Viewing window
Image 3: "show full rez browse" View
Highlights of Version 1.1 include:
About the Images:
- Data available through the previous day
- Multiple navigation methods
- Link to graphic display of pending acquisition locations
- Help, FAQ, and relevant links
- 3x3 regional "mosaic view" and single scene views
- Metadata subset display
- Images are bands 5,4,3
- sub-sampled to 240m resolution from the 30m resolution data
- JPEG compressed
- color-stretched on an individual scene-content basis
- Viewer displays browse images in Lambert Azimuthal Equal Area Projection
- Accuracy to within a few pixels at 240m resolution can displace data by over 1Km
- North is pointing from path to path-1, not necessarily towards the top
- Browsed images are not intended for analytical use
- UNIX or Windows Platform requirements: Netscape 4.7 or higher with Java 1.1 enabled
- PC Requirements: Internet Explorer 5.0 or higher with Java 1.1 enabled
- MAC: Internet Explorer 4.5 and Mac OS Runtime for Java(MRJ) 2.2
Some Facts About Landsat 7
LANDSAT 7 was launched on April 15, 1999 from the Western
Test Range at Vandenburg Air Force Base on a Delta-II launch
vehicle. At launch, the satellite weighed approximately 4,800
pounds (2,200 kilograms). The spacecraft is about 14 feet long (4.3
meters) and 9 feet (2.8 meters) in diameter. It consists of a
spacecraft bus, built by Lockheed Martin Missiles and Space in
Valley Forge, Pa., and the Enhanced Thematic Mapper Plus
(ETM+) instrument, developed by Raytheon Santa Barbara Remote
Sensing in Santa Barbara, Calif.
The satellite orbits the Earth at an altitude of approximately 438 miles (705
kilometers) with a sun-synchronous 98-degree inclination and a descending
equatorial crossing time of 10 a.m. LANDSAT Worldwide Reference System will
be maintained with periodic adjustments for the life of the mission. A three-axis
attitude control subsystem stabilizes the satellite and keep the instrument
pointed toward Earth to within 0.05 degrees. The LANDSAT
World-Wide-Reference system catalogues the worlds landmass into 57,784
scenes, each 115 miles (183 kilometers) wide by 106 miles (170 kilometers)
long. The ETM+ will produce approximately 3.8 gigabits of data for each scene,
which is roughly equivalent to nearly 15 sets of encyclopedias at 29 volumes per
set. (Source: USGS Landsat 7 homepage)
Coming with L7 Viewer Version 1.2
Joseph J. Kerski of the USGS Education Outreach program tells us that planned enhancements for Version 1.2 of the Landsat 7 Viewer
include listing of metadata, print/download graphics in an html pop-up window,
Toggle on/off display of state/country political boundaries, and full interaction while in single scene view.
Looking For More Imagery?
Not to be outdone, the CCRS in Canada has an equally (if not even more impressive) image browser -
The Quicklook Swath Browser.
The Quicklook Swath Browser is a viewer for Landsat 7, Radarsat, and NOAA quicklook browse imagery
acquired at CCRS. Crrently, the data is not in a geocoded format, but plans are in the works to serve it up via an OGC Web Map Server, likely later this year. Michael Adair, Geospatial Infrastructure Architect with The Canada Centre for Remote Sensing explains..."I feel that browse imagery could have a huge impact in the free imagery market, especially in the education community where funding seems to be perpetually in short supply. It could also have a big impact in Web Mapping where people are often only looking for "overview" type imagery and do not
plan on doing a great deal of analysis at high resolution."
Are you working with Landsat 7 imagery and would like to share some details of
a project your working on? Tell us about it!